Small fish make big splash in ocean carbon cycle

SUMMIT COUNTY — A still-popular first-grade book described the heroic efforts of a small fish to make a big splash. Now, it turns out that Arty’s dream wasn’t all that farfetched.

According to a new study by scientists with Rutgers University and the Virginia Institute of Marine Science, forage fish like anchovies can play an important role as a biological pump in the cycle that moves carbon dioxide from the atmosphere into the depths of the ocean, where its sequestered without adding to heat-trapping woes of atmospheric greenhouse gases.

Dr. Grace Saba, of Rutgers University, and professor Deborah Steinberg, of the Virginia Institute of Marine Science, shifted their focus away from their long-term studies of copepods to looking at anchovies in the Santa Barbara Channel, off the California coast.

“We collected fecal pellets produced by northern anchovies, a forage fish,” said Saba. By calculating that the pellets sink at an average rate of about 2,500 feet per day, they determined that pellets produced at the surface would travel to the seafloor at their study site in less than a day.

“’Fecal pellet’ is the scientific term for “poop,” Steinberg said. “Previous studies in our lab and by other researchers show that zooplankton fecal pellets can sink at rates of hundreds to thousands of feet per day, providing an efficient means of moving carbon to depth. But there have been few studies of fecal pellets from fish, thus the impetus for our project.”

Saba and Steinberg also counted pellet abundance — up to 6 per cubic meter of seawater, measured their carbon content — an average of 22 micrograms per pellet, and painstakingly identified their partly digested contents — mostly single-celled algae like dinoflagellates and diatoms.

“Twenty micrograms of carbon might not seem like much,” said Steinberg, “but when you multiply that by the high numbers of forage fish and fecal pellets that can occur within nutrient-rich coastal zones, the numbers can really add up.”

Saba and Steinberg calculate that the total “downward flux” of carbon within fish fecal pellets at their study site reached a maximum of 251 milligrams per square meter per day—equal to or greater than previously measured values of sinking organic matter collected by suspended “sediment traps.”

“Our findings show that—given the right conditions—fish fecal pellets can transport significant amounts of repackaged surface material to depth, and do so relatively quickly,” says Saba.

Those conditions are likely to occur in places like the western coasts of North and South America, where ocean currents impinge on continental shelves, bringing cold, nutrient-rich waters from depth into the sunlit surface zone.

The pair’s previous research showed that copepods and other small, drifting marine animals also play a key role in the biological pump by grazing on photosynthetic algae near the sea surface, then releasing the carbon they’ve ingested as “fecal pellets” that can rapidly sink to the deep ocean. The algal cells are themselves generally too small and light to sink.

The data was collected during an oceanographic expedition to the California coast during Saba’s graduate studies at VIMS. Saba, now a post-doctoral researcher in Rutgers’ Institute of Marine and Coastal Research, earned her Ph.D. from the College of William and Mary’s School of Marine Science at VIMS in 2009. The expedition, aboard the research vessel Point Sur, was funded by the National Science Foundation.